1. Field of The Invention
The present invention relates to a method and apparatus for enabling the modification or correction of printed circuit boards by adding interconnections to existing boards. It relates particularly to apparatus, herein called stitching-pads, for use on printed circuit boards to enable new electrical connections to be made. It relates further to apparatus and methods for fabricating such pads and for using such pads to provide bases on printed circuit boards to which connections may be welded in order to modify interconnections between existing electrical circuitry on such boards.
2. DESCRIPTION OF THE PRIOR ART
In order to correct an existing printed circuit board, or to implement engineering instructions for changes to such an existing printed circuit board, it is necessary either to change connections, as by adding or deleting connections, between certain connection points on the printed circuit board or to redo the artwork for the printed circuit board. Adding wire connections from point-to-point on a board appears, on its face, to involve simple and straight-forward procedures, but has not been done satisfactorily in the past. Redoing the artwork is expensive, in that existing inventory must be scrapped, and it is time consuming, in that normal lead times approximate 14-16 weeks. Redoing the artwork is, in any case, not practical for only one or a few boards.
A system currently on the market for making corrections to existing printed circuit boards by redoing the artwork is marketed by Additive Technology Corporation, a Massachusetts company. In that system the printed circuit board is re-etched on a dielectric applied over existing circuitry to implement the corrections or engineering instructions. That method is expensive in that it is priced at several dollars per board with an initial set-up charge of several hundred dollars. It has been determined that that system is not cost effective for runs of fewer than 50,000 boards. The turn-around time is too long for many purposes, since it is approximately three weeks.
A system for adding connections is marketed by Augat. This system employs a printed circuit board of special construction which is faced by a stainless steel sheet to which wire is stitched, as by welding. The wire utilized is formed of nickle with a teflon coating and the wire stitching employs resistive welding. FIG. 1 is a sectional view illustrating the application of this prior art system which involves three distinct steps, or procedures. In step 1 an upper Tungsten electrode 26 is lowered until the teflon insulation 24 encasing nickel wire 22 is in contact with an etched pad of stainless steel. In step 2 the upper electrode is advanced further against the teflon which breaks at its contact with the stainless steel as well as at its contact with the Tungsten edge. In step 3, a potential is applied across electrodes 26 and 28 causing the nickel wire and the stainless steel to be melted together to form a weld at 30 as a consequence of the excessive heat developed in the resistance at the nickel-stainless steel junction. It will be noted that the current flow between the aluminum electrode and the copper sheet causes little heat because of the high conductivity of both metals. In addition, it should be noted that the teflon flows back into the break at the top and thus is self-healing.
The printed circuit boards according to the Augat system with its special boards have application primarily in the prototype stage. Augat markets a small wire stitching machine for use with its printed circuit boards for approximately two to three thousand dollars which permits placement and attachment of wires by hand. A more expensive semi-automatic machine is available utilizing an x-y table for positioning the printed circuit board. However, the positioning of the wire, the cutting of the wire, the routing of the wire, and other similar operations are performed manually. This system is objectionable for use in the correction of production runs because it requires the use of special boards and is labor intensive.
Two patents which appear to relate to the subject matter of the Augat system have been granted to Larry A. Conley. These are U.S. Pat. Nos. 3,786,172 and 3,850,711. In each of these, a printed circuit board is disclosed which has both copper and stainless steel layers with segments of each exposed enabling solder to be applied on the copper surface, as by wave soldering, and welding to be performed on the stainless steel surfaces. Clearly, a system using this special printed circuit board will not be useful for the correction of other types of existing boards.